Process for producing a material composite, material composite and use of the material composite as a heat conductor and heat exchanger

ABSTRACT

Processes produce a compound material structure by producing a composite material which extends along an axis of elongation from carbon nanostructures anchored in a matrix of a first metal extending along the axis of elongation of the composite material. The processes comprise dividing the composite material into segments of the composite material, arranging the segments in a plane of a die matrix, filling free spaces in the die matrix with a filler material and subsequently sintering in the die matrix to form a compound material structure or squeeze casting in the die matrix, and exposing the carbon nanostructures of the composite material on at least one surface of the compound material structure such that the carbon nanostructures protrude out of this surface. Compound material structures and uses thereof as a heat conductor and/or a heat exchanger are also provided.

Process for the production of a compound material structure, a compound material structure as well as use of the compound material structure as a heat conductor as well as a heat exchanger

The invention relates to a process for the production of a compound material structure, a compound material structure, as well as to the use of the compound material structure as a heat conductor as well as a heat exchanger.

Wherever heat is generated in electronic components as a result of power loss, it also has to be dissipated in order to prevent the components from overheating. There are many applications in the prior art which benefit from increasing the flow of heat between two surfaces. In spacecraft in particular, in which no convection can take place because of the environmental conditions, wired heat transport between two surfaces in particular is decisive. The temperature in the components can be controlled better by means of enhanced thermal connection of the components to the remaining satellite bus and in particular to radiators. When surfaces are connected together, there is a flow of heat between them which is a function, inter alia, of the contact surface area, the roughness, the contact pressure and the material properties. The effective contact surface area is significantly reduced here because on a microscopic level, the surfaces are not flat. This is illustrated in FIGS. 1 and 2 with the aid of two different contact layers 1 and 2. Polishing or lapping the surface is possible in order to increase this surface area, but this still leaves a residual microscopic roughness.

However, the flow of heat between the surfaces does not only occur via the contact surfaces, but also via the gaps between the surfaces by radiation or thermal conduction or convection of the medium which is between them. However, in a vacuum, there is in fact no convective heat conduction.

In order to increase the heat conduction between two surfaces, to the present date, various “Thermal Interface Materials” (TIMs) have been developed with which the gaps are filled.

In this regard, it is standard for heat conducting gels, pastes or other materials which are partially based on carbon to be used here which, however, cannot as a rule be recycled but have to be changed when a fresh contact is made.

Carbon nanostructure arrays lend themselves for use as Thermal Interface Materials because the carbon nanostructures, preferably carbon nanotubes (CNT), have a thermal conductivity in their direction of growth of up to 3500 W/m K. Such an option for an interface based on carbon nanotubes as “Thermal Interface Materials” is proposed in US patent U.S. Pat. No. 7,416,019. In it, the carbon nanotubes are fastened to the surface or are grown on the surface of a metal.

The aim of the invention is to provide a reusable and effective interface for conducting or exchanging heat between two surfaces.

The process for the production of a compound material structure in accordance with the invention generally comprises the following steps: production of a composite material which extends along an axis of elongation, produced from carbon nanostructures anchored in a matrix of a first metal; dividing the composite material into segments, in particular by sawing, for example along or perpendicular to the axis of elongation of the composite material; arranging the segments in a plane of a die matrix; filling free spaces in the die matrix with a filler material, sintering in the die matrix to form a compound material structure, exposing the carbon nanostructures of the composite material from at least one surface of the compound material structure, so that the carbon nanostructures protrude out of this surface and still in part remain anchored in the base material.

This process has the advantage that on the one hand, as a result of the protruding carbon nanostructures, the contact surface area between two surfaces is increased, and on the other hand, because the carbon nanostructures are embedded in the metal matrix in a stable manner, an interface produced from a compound material structure of this type can be configured so as to be releasable.

The term “carbon nanostructures” as used below should be understood to mean structures such as round carbon nanoparticles such as fullerenes and amorphous carbons, for example, or layered carbon nanoparticles such as graphites and nanoplatelets, for example, or fibrous carbon nanoparticles such as carbon nanotubes and carbon nanofibers, for example. Preferably, the carbon nanostructures are carbon nanotubes.

In this manner, the invention allows the interface surface area to be increased and/or the contact surface area of a releasable and reusable thermal interface to be increased, whereupon the flow of heat between two surfaces is increased.

The carbon nanostructures may extend in the metal in a randomly distributed manner. In a preferred exemplary embodiment, the carbon nanostructures extend along the axis of elongation of the composite material. After exposing the carbon nanostructures, they then protrude out of the surface of the composite material, preferably in one direction. This provides improved contact, improved heat transfer and improved reusability of the interface.

In particular the composite material may be a rod-shaped composite material and the cross-sectional surface of the rod-shaped composite material may have any basic geometrical shape, in particular a circular, trapezoidal, rectangular or square basic shape, or it may be formed from circular segments.

Preferably, the process comprises the following steps which follow sintering in the die matrix: shaping the sintered body by forming, for example by extrusion, ECAP (Equal Channel Angular Pressing) or rotary swaging, machining, and grinding the surface of the composite material from which the carbon nanostructures are to be exposed.

The production of the composite material is preferably carried out by powder metallurgy and comprises the following steps: production of a homogeneous powder mixture from a first metal and from carbon nanostructures, sintering the powder mixture to form a composite material, and extrusion of the composite material. Direct extrusion of the homogeneous powder mixture is also possible.

The carbon nanostructures are preferably exposed to a length of 5-30 μm, more preferably 10-20 μm.

The first metal is preferably copper. However, any other metal may also be used.

Thus, the invention proposes an increase in the interface surface and/or contact surface area of a releasable and reusable thermal interface for increasing the flow of heat between two surfaces produced from metal-carbon composite materials, in particular from copper—carbon nanostructures, by the formation of a compound material structure, in particular via copper or copper—carbon composite materials for different atmospheres, preferably in vacuum in a pressure zone of less than 1*10⁻² mbar.

The filler material preferably has a higher thermal conductivity than the composite material. In this manner, the overall thermal conductivity can be improved. The filler material can be introduced using powder metallurgy and/or smelting metallurgy methods.

In particular, the filler material comprises a second metal. This may be copper. The filler material may be a metal-carbon composite material. Metal-diamond composite materials are possible, preferably copper-diamond, or a metal-graphite composite material, preferably copper-graphite. These materials are particularly suitable for improving the thermal conductivity.

In one exemplary embodiment, at least one first layer of at least one other material may be introduced into the die matrix in the plane of the composite material. As an alternative, or in addition, prior to the step of introducing the segments into the die matrix, the die matrix may already have been filled with at least one second layer of at least one other material and the segments can be disposed thereon. The first and second layers preferably have a higher thermal conductivity than the composite material.

In this manner, an interface produced from the compound material structure can be specifically adapted to the dimensions of components and the thermal conductivity requirements.

Furthermore, the invention encompasses a compound material structure which has been produced in accordance with the invention as described above.

Furthermore, a use of a compound material structure in accordance with the invention as a thermal conduction material and heat exchange material is proposed.

The properties, features and advantages of this invention as well as the ways of generating them will become clearer and more comprehensible from the more detailed description made in association with the following description of exemplary embodiments which are explained in more detail in association with the drawings, in which:

FIG. 1 diagrammatically shows a non-contacted thermal interface of the prior art and consists of two contact layers,

FIG. 2 diagrammatically shows a contacted thermal interface of the prior art of FIG. 1,

FIG. 3 shows the composite material in accordance with the invention after extrusion, with exposed carbon nanostructures,

FIG. 4 diagrammatically shows a non-contacted thermal interface with a composite material in accordance with the invention, in a first exemplary embodiment,

FIG. 5 diagrammatically shows a contacted thermal Interface from FIG. 4,

FIG. 6 diagrammatically shows a second exemplary embodiment of a non-contacted thermal interface in accordance with the invention,

FIG. 7 diagrammatically shows a further possible arrangement of a thermal interface in accordance with the invention,

FIG. 8 shows, by way of example, a possible rod of composite material after extrusion,

FIG. 9 shows a cropped segment of the extruded composite material,

FIG. 10 shows, by way of example, the arrangement of several segments in the die matrix, in preparation for sintering,

FIG. 11 shows a compound material structure in accordance with the invention, mechanically and thermally composited by sintering,

FIG. 12 shows an exemplary/diagrammatic representation of a machined, ground and etched compound material structure in accordance with the invention,

FIG. 13 shows a representation of a compound material structure which has been produced and ground,

FIG. 14 shows a representation of a produced, ground and etched compound material structure for verification of the process,

FIG. 15 diagrammatically shows an embodiment in accordance with the invention of a thermal interface connected to a body with a material with a lower, identical or higher thermal conductivity,

FIG. 16 diagrammatically shows an embodiment in accordance with the invention of a thermal interface connected to a body with a material with a lower thermal conductivity and an identical or higher thermal conductivity at several sites for the formation of specific heat conduction pathways,

FIG. 17 diagrammatically shows the process in accordance with the invention for the production of a compound material structure.

FIG. 1 shows a non-contacted thermal interface of the prior art. Here, the thermal interface consists, for example and in a non-limiting manner, of a respective metallic contact layer 1 and 2 which each have microscopically roughened surfaces which face each other. If these two surfaces are brought together into contact, as can be seen in FIG. 2, this results in an effective surface area for contact-associated heat exchange from the sum of the points of contact 3 between the contact layers 1 and 2. The heat can only be transferred via the gaps 4 between the points of contact 3 by radiation or convection of the enclosed medium. However, in a vacuum, convection cannot occur.

Thus, in accordance with the invention, a compound material structure produced from metal-carbon composite materials is proposed, in particular produced from copper and carbon nanostructures such as, for example, carbon nanotubes, but not limited thereto. In the composite material, the carbon nanostructures are anchored in a metal matrix. In this manner, they protrude out of a surface and thus can be used for a thermal interface, as “Thermal Interface Materials” (TIM).

In this regard, the metal-carbon composite material is produced by powder metallurgy. A first metal acts as the matrix and the carbon is primarily acting as a reinforcing component. Advantageously, there is a variety of possibilities for subsequent shaping of the composite material. As an example, but not limited thereto, after the production of a homogeneous powder mixture, the metal-carbon nanostructure composite material can be provided with a shape, in particular by extrusion. In this regard, the carbon nanostructures, preferably carbon nanotubes, are orientated substantially parallel to the extrusion direction in one dimension. After extrusion, the composite materials can be machined as normal. Thus, the surface can brought to the shape that is suitable for the thermal interface and be made smaller by processes such as lapping to a roughness of up to 10 μm, preferably up to 1 μm and below.

By etching away the uppermost metal layer on the end face, the carbon nanostructures which were embedded therein can be exposed, preferably over a length of up to 10 μm, more preferably up to 20-30 μm. The carbon nanostructures which therefore protrude out of the surface are still firmly anchored in the metal matrix. A composite material 20 of this type after extrusion is shown in FIG. 3. After extrusion, the carbon nanostructures 22 are exposed by etching away a first metal 24 at the surface. Because the carbon nanostructures 22 are anchored in the first metal 24, preferably copper, the carbon nanostructures 22 are not as easy to detach from the corresponding contact layers 1 and 2 upon separation. As a result of this, the composite material 20 is better suited to releasability and reusability of the interface. The surface of the composite material 20 has zones of the first metal 24 through which the carbon nanostructures 22 pass or protrude from the surface. In FIG. 3, the composite material 20 extends along an axis of elongation in the z-direction. The side face(s) of the composite material 20 are formed from the first metal 24. However, these sides can also be etched away.

FIG. 4 diagrammatically shows a non-contacted thermal interface which, by way of example, has a metallic contact layer 1 on one side and a contact layer 2 consisting of a metal-carbon nanostructure composite material 20 as described above on the other side. The carbon nanostructures 22 of the front face of the composite material 20 have been exposed here by etching, by way of example. FIG. 5 now diagrammatically shows the contacted thermal interface of FIG. 4. In this regard, the number of contact points 3 compared with the number of contact points 3 in FIG. 2 is significantly higher because of the carbon nanostructures 22 embedded in the first metal 24.

FIG. 6 diagrammatically shows a further exemplary embodiment of a non-contacted interface in accordance with the invention. This now consists of two respective metal—carbon nanostructure composite materials 20 as described above with exposed carbon nanostructures 22. In the contacted state, the respective carbon nanostructures 22 touch the surface produced from the first metal 24 or the carbon nanostructures 22 of the other contact layer. In this manner, the thermal conductivity is increased still further. FIG. 6 shows carbon nanotubes 22 purely by way of example. However, these may also be any other carbon nanostructure 22 such as round carbon nanoparticles, for example fullerenes and amorphous carbon materials, or layered carbon nanoparticles, for example graphites and nanoplatelets, or fibrous carbon nanoparticles, for example carbon nanofibres. Preferably, however, and in a non-limiting manner, the carbon nanostructures are carbon nanotubes.

FIG. 7 diagrammatically shows a further possible arrangement of the thermal interface. In addition to the variations discussed above, here, the surfaces with and without carbon nanostructures 22 on both sides of the thermal interface are respectively offset with respect to each other. By means of the offset arrangement of the regions with and without carbon nanostructures on the two contact layers 1 and 2, releasability of the interface can be improved.

The contact surfaces which can be produced and a possible shape for the thermal Interfaces are limited in the process which produces the composite material 20. In order to expand this and provide an adaptive design, a production process for the production of a compound material structure is proposed which makes it possible to connect together mechanically and thermally interface elements which have been produced by the process described above.

As a result of this, the contact surface areas of an interface body can be increased and can be made into any shapes. In this manner, in particular, interface rings produced from circular segments can be produced.

To this end, a powder metallurgy process in accordance with the invention will now be described.

FIG. 8 shows, by way of non-limiting example, a rod-shaped composite material 20 with a cross-sectional surface 26 produced from a circular segment following an extrusion step. The cross-sectional surface 26 of the rod-shaped composite material 20 can in fact have any basic geometrical shape, in particular a circular, trapezoidal, rectangular or square basic shape. This rod extends along an axis of elongation z and in the example shown, is used for the preparation of circular segments in FIG. 9. The carbon nanostructures 22 also extend along the axis of elongation z out of a surface of the composite material 20. Preferably, the composite material 20 is in the shape of a rod, so that it can be partitioned easily, for example by sawing. The rod which is produced by extrusion is divided into segments 30 of an appropriate thickness, but preferably not limited to sawing. These segments 30 are shown in FIG. 9. The shape and outline of the composite material 20 is not limited to that shown in the exemplary embodiment. The outline may have any shape, for example square, rectangular, circular, elliptical, etc. Production may be such that a sheath produced from a first metal 24 is formed around the composite material 20 and which can be machined away if required. This sheath may, however, also be used for further joining (for example by soldering).

The segments 30 are disposed in a die matrix 100 with a selected shape. FIG. 10 shows, by way of non-limiting example, that a circular shape has been selected as the die matrix 100. The segments 30 are disposed in the die matrix so that they form a circular ring 110 and the free spaces 120 that are between them are then filled with a filler material 130, as shown in FIG. 11. The filler material 130 is preferably a metal powder, more preferably copper powder. In the exemplary embodiment shown, in the interior of the die matrix 100 there is no composite material 20 around the centre of the circle; only the filler material 130 is disposed there. The filler material 130 and the segments 30 are connected together, for example by sintering, to form a common body 200.

Particularly in the case of circular rings, the inner region may be filled with the filler material 130 in order to be used as a clamping area for later machining. After sintering, the compound material structure can be machined to the final shape.

In one exemplary embodiment, segments of the composite material may be disposed in a plane in a die matrix which can be used as a squeeze casting tool. The die matrix is then pre-heated to temperatures between 400° C. and 600° C., preferably in a vacuum. As an example, molten metal or a metal alloy, for example copper at a temperature between 1200° C. and 1300° C., for example under vacuum (<20 bar) and at a predetermined pressure, then penetrates into the voids between the segments. The predetermined pressure may be between 50 MPa and 100 MPa, for example approximately 80 MPa. The penetration period may be between 35 and 50 seconds. Next, solidification is carried out under pressure. The compound material structure can then be pushed out of the die matrix and can cool further in air.

FIG. 12 shows a representation of a machined, ground and etched compound material structure 200 which has been produced by the aforementioned steps. As an example, the compound material structure 200 is formed as a ring and has individual holes in the ring which can be used for fastening. The last steps of the process are grinding the surface of the compound material structure, the result of which is shown in FIG. 13, as well as etching in order to expose the carbon nanostructures (FIG. 14).

The interface can be used both when the carbon nanostructures 22 are exposed on one side or in fact on both sides of the contact surfaces. It is also possible to use it against another solid material.

The thermal conductivity of the compound material structure 200 can therefore be adjusted in this manner. Metal-diamond or metal-graphite composite materials have a higher thermal conductivity than the pure metal or the metal—carbon nanostructure composite material 20. The thermal conductivity of copper-diamond is up to 700 W/m K and that of copper-graphite is up to 600 W/m K, while the thermal conductivity of pure copper is approximately 400 W/m K. Thus, they can also be used for passive cooling. This can also be employed for this invention. In order to increase the thermal conductivity, in particular, the filler material 130 for connecting the compound material structure can be substituted by a metal-diamond composite material. Metal-diamond composite materials are distinguished by a higher thermal conductivity in all directions in space compared with pure metal, whereupon the entire amount of heat to be exchanged can be increased still further.

In this manner, the compound material structure 200 can be shaped and adapted in many ways. FIG. 15 diagrammatically shows a compound material structure 200 with a first subsection 210 produced from the composite material 20, connected to a body with a second subsection 220 which is formed from a material with a lower, identical or higher thermal conductivity. The connection may also be produced by sintering or squeeze casting. The material of the second section 220 may be disposed in the die matrix 100 together with the segments 30 of the composite material 20. By selecting a material with a higher or lower thermal conductivity for the second subsection 220, specific heat conduction pathways can be formed. The rigidity and strength can also be raised in this manner.

A more complicated exemplary embodiment of a compound material structure 200 is shown in FIG. 16. Here, the compound material structure 200 consists of two plies of different materials. Thus, for example, a third section 230 may be disposed below the composite material 20 and have a lower thermal conductivity than the composite material 20 of the first section 210. The fourth section 240 may have an identical or, preferably, higher thermal conductivity, whereupon the heat is conducted away laterally.

FIG. 17 shows a process in accordance with the invention for the production of a compound material structure. In step S100, initially, a composite material 20 is produced from carbon nanostructures 22 anchored in a matrix of a first metal 24. Here, the composite material 20 extends along an axis of elongation z. The carbon nanostructures 22 also extend along the axis of elongation z of the composite material 20. In step S200, the composite material 20 is divided into segments 30, preferably cut segments 30. In this manner, the segments 30 are disposed in at least one plane in a die matrix 100, at S300. In step S400, a compound material structure 200 is then formed. This may be carried out by filling, at S410, the free spaces 120 in the die matrix with a filler material 130 and subsequent sintering, at S420, in the die matrix 100. As an alternative, this may also be carried out by squeeze casting, at S430, in the die matrix 100. Next, in step S500, the carbon nanostructures 22 on at least one surface of the compound material structure 200 are exposed, so that the carbon nanostructures 22 protrude out of this surface.

In summary, carbon nanostructures anchored in a metallic matrix are proposed as Thermal Interface Materials (TIM) and heat exchange materials. These can advantageously be used in a releasable and reusable thermal interface. In accordance with the invention, a compound material structure can be produced by sintering, for local integration of the thermal active interface surface into a metal, a metal alloy and/or a composite material (metal/diamond, metal/graphite).

In addition, specific thermally active interface surfaces made from metal/carbon nanostructure composite materials for heat exchange and a material with a lower thermal conductivity (ceramic, metal, metal alloys and composite materials) as the composite material for the formation of specific conducting pathways (thermal partitioning) can be constructed.

Advantageously, the thermally active interface surface can be regenerated by spe fic etching and can be adjusted to the contours.

Although the invention has been illustrated and described in detail with the aid of preferred exemplary embodiments, the invention is not limited to the examples disclosed and other variations can be envisaged by the person skilled in the art without departing from the scope of the invention.

LIST OF REFERENCE NUMERALS

-   1 first contact layer -   2 second contact layer -   3 contact points -   4 gaps between contact points -   20 composite material -   22 carbon nanostructures -   24 first metal -   26 cross-sectional surface -   30 segments -   100 die matrix -   110 die matrix segment -   120 free space -   130 filler material -   200 compound material structure -   210 first section -   220 second section -   230 third section -   240 fourth section 

1. A process for producing a compound material structure the process comprising: producing a composite material extending along an axis of elongation (z) from carbon nanostructures anchored in a matrix of a first metal; dividing the composite material into segments; arranging the segments in at least one plane in a die matrix; forming a compound material structure by filling free spaces in the die matrix with a filler material and subsequent sintering in the die matrix, or squeeze casting in the die matrix; exposing the carbon nanostructures out of at least one surface of the compound material structure such that the carbon nanostructures protrude out of the at least one surface of the compound material structure.
 2. The process according to claim 1, wherein the carbon nanostructures are round, layered, or fibrous carbon nanoparticles.
 3. The process according to claim 1, wherein the composite material is a rod-shaped composite material and the cross-sectional surface of the rod-shaped composite material has a basic geometrical shape comprising, a circular basic geometrical shape, a trapezoidal basic geometrical shape, a rectangular basic geometrical shape, or a square basic geometrical shape or subsections of the basic geometrical shape.
 4. The process according to claim 1, further comprising shaping by machining; and grinding the at least one surface from which the carbon nanostructures are to be exposed, wherein both the shaping and grinding are carried out after the die matrix has been sintered.
 5. The process according to claim 1, wherein the producing of the composite material is carried out by powder metallurgy and comprises: producing a homogeneous powder mixture from the first metal and the carbon nanostructures; and sintering the homogeneous powder mixture to form a composite material; and/or extruding the composite material.
 6. The process according to claim 1, wherein the carbon nanostructures are exposed over a length of 5-50 μm on the at least one surface of the compound material structure.
 7. The process according to claim 1, wherein the first metal is copper.
 8. The process according to claim 1, wherein the filler material has a higher thermal conductivity than the composite material.
 9. The process according to, wherein the filler material: comprises a second metal; is copper; is a copper-diamond composite material; or is a copper-graphite composite material.
 10. The process according to claim 1, wherein at least one first layer of at least one other material is introduced into the die matrix in the plane of the composite material.
 11. The process according to claim 1, wherein, prior to the introduction of the segments into the die matrix, the die matrix was filled with at least one second layer of at least one other material and the segments are disposed thereon.
 12. The process according to claim 11, wherein the at least one first and at least one second layers have a lower or higher thermal conductivity compared with the composite material such that one or more heat conduction pathways are formed.
 13. A compound material structure obtained by the process according to claim
 1. 14. A method comprising: conducting or exchanging heat between two surfaces via a reusable and effective interface comprising the compound material structure according to claim
 13. 15. The process according to claim 6, wherein the carbon nanostructures are exposed over a length of 10-30 μm on the at least one surface of the compound material structure. 